(1) Field of the Invention
The invention relates to processes for packaging MEMS devices, and MEMS packages produced using the method, and more particularly, to a method of packaging MEMS devices for enhanced EMI immunity using a flexible, foldable substrate.
(2) Description of the Related Art
Micro-electro-mechanical systems (MEMS) devices are known which convert physical phenomena, such as pressure, acceleration, sound, or light, into electrical signals. Each type of MEMS device interacts with the world in a different way, and demands custom or at least semi-custom packaging solutions. So-called system-in-package techniques attempt to form an entire microsystem—which could include a microprocessor, communications components, actuators and sensors—within a single package. However, packaging of a MEMS device is totally different from packaging an integrated circuit. MEMS devices are categorically different from ICs despite sharing some fundamental processing technologies. Packaging is the biggest challenge for commercializing most MEMS devices. The term “MEMS package” is used in this document to imply a package including at least one MEMS device.
A MEMS device might function perfectly well in the controlled environment in which it was created. However, the device can be a real viable product only after it has been packaged with proven performance in a package. For example, the packaging stress can distort the sensitivity and the performance of the MEMS devices. MEMS devices include delicate movable structures which are easily damaged through fabrication and assembly processes. As such, the assembly yield of a MEMS package is often a challenging target to meet.
The packaging requirements of MEMS devices are complex because the devices need to interact with the physical phenomenon and yet the devices need to be protected from the environment. As such, exotic package structures with specialized assembly techniques and unique packaging materials are employed for MEMS devices.
U.S. Pat. No. 6,324,907 to Halteren et. al. discloses a flexible substrate assembly in which a transducer element and other signal processing devices can be integrated and encapsulated with a metal cap. The flexible substrate has the elongation to provide easy connectivity between the transducer assembly and compact electronic equipment like mobile phone and hearing aid products.
U.S. Pat. No. 6,781,231 to Minervini teaches a method of packaging a MEMS microphone into a house formed by a substrate having at least one conductive layer and a conductive cap. The conductive layer and cap are electrically connected to shield the MEMS acoustic transducer from external EMI.
U.S. Patent Application No. 2002/0102004 A1 to Minervini discloses a MEMS microphone package in which a MEMS transducer element, an IC die and other capacitor components are located on a first multi-layer FR4 printed circuit board (PCB). A second multi-layer FR4 PCB is used as a cover. The two FR4 boards are spaced apart by a third FR4 board, which is cut to include a window which is placed around the components on the first PCB. Thus, the three PCBs cooperate to house and shield the transducer element, the IC die and other capacitor components. Compared with plastic/lead frame packages, such a package enables a larger batch operation, requires minimal hard tooling and has better match of thermal expansion with end user's PCB.
PCT patent application PCT/SG2005/000034 filed on 8 Feb. 2005 provides a method and package in which at least one MEMS device is mounted on a first flex substrate, and one or more electronic components are mounted on a second substrate. The two substrates are then joined mechanically in parallel with a spacer element between them and connected electrically by electrical connecting elements. The substrates sandwich the spacer element, the electrical connecting elements, the MEMS device and the one or more electronic components between them. The advantage of this method is that the process of mounting the MEMS device can be dealt with and performed separately from the process for mounting other IC and electronics components, thus making the assembly easier and higher yield. However, interconnecting the two substrates is a challenging task.
U.S. patent application Ser. No. 11/182254, filed on Jul. 15, 2005 teaches the methods of using flexible substrates and folding processes to make MEMS packages.
However, ubiquitous electromagnetic interference (EMI) leakage is a major noise source to most electronic devices, especially for acoustic MEMS devices because of required openings for sound inlet. EMI signals can easily find their ways to go through the environmental hole or through overlapping gaps between metal layers on the substrate, for example. FIG. 1 illustrates a MEMS package including MEMS device 40 and other devices located on a substrate 10 including a grounded metal layer and covered by a grounded metal cap 54. EMI leakage can occur through environmental hole 11 and through overlapping gaps 15. EMI noise is represented by 17 in the figure.
U.S. Patent Application 2005/0018864 to Minervini shows a cavity under the MEMS device in a multi-layer1 PCB. A polymeric layer is adopted as an environmental barrier within the acoustic aperture for preventing moisture, dirt and/or dust from entering the packaging house. U.S. Patent Application 2005/0135727 (Piede et al) shows opto-electronic components on an SOI layer. EMI shielding is provided by a metallized outer layer plated on the SOI over EMI-sensitive electronics and coupled to an external ground plane.